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Their results change the probability of disease Negative testPositive test Reassurance Treatment Order a Test A good test moves us across action thresholds. 0%100% T+ T- The best tests are definitive What tests do

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Post-Test Probability of Disease Depends on 2 Things 1. Where you started from (low, medium, high) 2. Length and direction of the arrow Basic paradigm: What we thought before + test result = what we think now Prior probability + LR from test = post-test probability LR = P(Result|Disease)/P(Result|No Disease)

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Whats wrong? StrepNo StrepTotal Rapid Test + TPFPTP+FP Rapid Test - FNTNTN+FN TP+FNFP+TN 2 definitions of false negative rate 1-sensitivity = FN/(TP+FN). This one is easier because its (assumed to be) constant. 1 - negative predictive value = FN/(FN+TN). This one is harder because it depends on prior probability, but it is the one that should determine clinical decisions.

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Similar examples: Sensitivity of UA for UTI is only 80%, therefore always culture after a negative UA Sensitivity of CT scan for subarachnoid hemorrhage is only 90%, therefore always do LP after a negative CT